The use of jet pumps for circulating a coolant fluid, such as water, through the fuel core of a nuclear reactor contained within a pressure vessel is well known as shown in U.S. Pat. Nos. 3,378,456; 3,389,055; 3,625,820 and 3,838,002 which are incorporated by reference herein.
A jet pump utilizes a stream of driving or motive fluid of high velocity to drive a stream of driven or suction fluid by momentum exchange between the two streams. In a known jet pump configuration a highly convergent suction inlet section is submerged in the driven fluid. A nozzle is positioned adjacent the suction inlet section, the nozzle receiving high-pressure driving fluid and directing a high-velocity stream of driving fluid into the suction inlet section by which driven fluid is entrained and driven into an elongated throat or mixer section where momentum is exchanged between the driving and driven fluids. The resulting flow exits the mixer section and enters a divergent diffuser section wherein flow velocity decreases and pressure increases.
In a typical nuclear reactor installation, as shown in FIG. 2B of U.S. Pat. No. 3,389,055, the jet pumps are vertically mounted and downwardly directed in a fluid downcomer annulus surrounding the nuclear reactor core whereby their operation pressurizes the coolant in a chamber beneath the core to thereby drive the coolant upward through the fuel assemblies of core.
As discussed in U.S. Pat. No. 3,389,055 and shown in FIG. 5 thereof, the mixer and diffuser sections are joined by a clearance fit slip joint. This facilitates the removal during reactor shutdown of the mixer section and other upper parts of the jet pump, which are subject to greatest wear, for inspection and repair or replacement as required. The slip joint also eliminates stress from thermal expansion differences between the carbon steel pressure vessel and the stainless steel jet pumps and core support structure.
Because there is a pressure differential across the slip joint, a leakage flow of fluid therethrough results. A limited amount of leakage flow through the slip joint is desirable to clean the joint and prevent crud and corrosion product buildup therein which might freeze the joint.
However, excessive leakage flow is found to cause undesirable jet pump vibration. Therefore, it is necessary to limit the leakage flow to a rate below that which induces damaging vibration.
A possible way to limit leakage flow is to reduce the annular clearance of the slip joint. However, this would require tighter manufacturing tolerances with attendant high cost and also increase the possibility of joint freezing. Another possibility is the use of some sort of sealing ring. This is undesirable because of the difficulty of remote handling of such parts and because of the possibility of breakage in use with the danger that broken parts might be carried into and become lodged in the reactor core. Furthermore it is desirable to provide leakage flow limiting without additional parts and without increase in the size of the parts.
Thus it is an object of the invention to limit leakage flow through the slip joint of a jet pump of a nuclear reactor without substantial additional cost or complication. Another object is to reduce leakage flow through the mixer-diffuser slip joint of a jet pump without additional parts or increase in size of the jet pump parts. A further object is reduction of leakage flow induced vibration of a jet pump to an acceptable level.